The structures of the mRNA and the gene for opsin are being examined. Retinal mRNAs from rat, bovine, monkey, and human eyes were extracted. The cDNAs made from purified mRNAs were cloned into the bacterial plasmid pBR322, and rat and bovine cDNA libraries were established. Bovine opsin cDNA was obtained from Dr. M. Applebury at Purdue University and used to screen the rat cDNA and human genomic libraries. We obtained several putative human opsin genes. These are being sequenced in order to establish their primary gene structure. Northern blot hybridization showed that bovine, rat, monkey, and human opsin mRNAs cross-hybridize and are 2,500 to 3,000 nucleotides long. These results indicate that opsin mRNAs are evolutionarily conserved, with respect to size and sequence homology. Intensities of hybridization suggest that bovine opsin mRNA is more similar to that of the rat than to that of the monkey or human. The large size of the opsin mRNAs indicates that their noncoding sequence is about twice the length of its coding sequence. The recombinant DNA probes for opsin will be used for detailed characterization of the structure, expression, and evolution of the genes for photopigments and for analysis of retinal disease, especially hereditary disorders affecting photoreceptors. Interestingly, Gamma-crystallin was shown to form a Schiff's base with retinal, as does opsin. Further investigations will determine the extent to which retinal proteins and lens proteins have related structures.